Various methods for measurement of bone characteristics using acoustic techniques are known for identifying patients in need of treatment for bone conditions and diseases. Many acoustic techniques utilize a first (transmitting) transducer to provide an acoustic signal, typically at ultrasonic frequencies, to the subject from a first external location and a second (receiving) transducer at a second external location disposed on the opposite side of the bone of interest to receive the signal transmitted by the first transducer through the bone and intervening soft tissue. (The transducers are typically coupled to the subject through a suitable fluid, such as water or water gel.)
One often-quoted acoustic characteristic of bone is the so-called Broadband Ultrasound Attenuation (BUA), typically quoted for the frequency range of approximately 300 to 700 kHz. The BUA is defined as the slope of a linear logarithmic-amplitude versus frequency plot of the energy transmitted through the heel. BUA measures are typically performed by Fourier transforming the signal produced in the receiving transducer due to transmission of a broadband acoustic pulse through the bone undergoing measurement. The Fourier components of the received signal are typically ratioed to the corresponding components measured through a medium of known or assumed spectral attenuation characteristics so that the slope of the bone attenuation versus frequency may be derived. Bone is known to have the effect of preferentially attenuating higher frequencies--this preferential attenuation is known to decrease as the bone becomes increasingly porous, thus the BUA similarly decreases for more porous bone. BUA measurements typically suffer from several drawbacks, including contamination of the received signal by acoustic transmission between the transmitting and receiving acoustic transducers via multiple paths through the bone. For example, the BUA result for a given measurement may depend not only on the apparatus used but on the length and portion of the time domain record used, the type, if any, of window function used with the data, the frequency range and method used for estimation of the slope, and the methods used for calibration.
The early portion of the received waveform may be more representative of the measured body part, as discussed in greater detail in the description below. It is desirable, moreover, to convert measurements made with respect to early or other transient portions of the received signal to equivalent measurements which might have been made using BUA techniques under ideal conditions and to quote the results in terms of a BUA-equivalent, since empirical data relate BUA values to bone condition.
For clinical utility, measured and quoted characteristics must be highly reproducible both across population samples and from measurement to measurement within an individual, whether the measurements are made with one or more than one measurement unit.
In order to monitor the reliability and repeatability of the measurements, standards of various sorts have been provided to simulate the attenuation properties of bone, namely preferential attenuation of higher frequencies. One type of standard requires fabrication of a model heel structure, such as an epoxy-resin phantom, sometimes filled with particles of another material such as tungsten powder or glass beads, and sometimes temperature-regulated. These standards are known to suffer from lack of repeatability in production and application. Another type of standard known in the art is an electronic standard which simulates the spectral effect of an attenuating bone, but fails to account for mechanical aspects of the overall system, such as the transducer sensitivity to contact effects.